6.2SDN architecture and the fabric

Cisco systematizes the controller-based idea from the previous section as SDN (Software-Defined Networking). The crux of SDN is separating a logical layer—policy and virtual networks—laid over the "foundation" of physical cabling. This section organizes the relationship between those two layers (underlay and overlay) and the fabric that binds them, what APIs the controller uses to talk to higher-level applications and to a neighboring controller, and what Cisco's implementation—SD-Access and Catalyst Center—handles, all while avoiding term confusion.

6.2.1Underlay, overlay, and fabric

• The underlay is the physical foundation network of switches/routers and cabling. It is the base that provides IP reachability between the fabric's nodes, usually built as a simple routed IP network.
• The overlay is a logical network built with tunnels (e.g., VXLAN) on top of the underlay. It lets you layer virtual segments and policy without changing physical cabling, and user traffic flows over this logical layer.
• The fabric is the whole—underlay plus overlay—treated as a single managed entity. The controller treats the entire fabric as one system and distributes policy to its nodes.